1. Field of the Invention
The present invention relates to an automatic speed control system for vehicle, and more particularly to a so called autocruising system in which a vehicle speed is maintained at a substantially constant value.
2. Description of the Prior Art
There has been known a vehicle which is controlled to run at a predetermined constant speed based on a difference between an actual vehicle speed and a target vehicle speed as shown by Japanese Patent Public Disclosure No. 59-192114, laid open to the public in 1984. In this type of vehicle speed control system, when the difference between the actual speed and the target speed is in a predetermined range, a shift up operation of a transmission is made for improving the fuel consumption efficiency. On the other hand, when the difference between the actual speed and the target speed is out of the predetermined range, a shift down operation is made for acceleration and deceleration.
The target vehicle speed can be reset when a driver of the vehicle desires more speed or less speed of the vehicle by using a set switch and a coast switch as shown by Japanese Patent Public Disclosure Nos. 61-238518 and 61-238515, laid open to the public in 1986.
U.S. Pat. Nos. 4,697,478 and 4,709,595 disclose conventional transmissions, which are assigned the same assignee as the present application.
It should be noted that it is not necessary to make a shift down operation of the transmission when the set switch and the coast switch are actuated for acceleration and deceleration. Unnecessary shift down operation causes a shift shock and causes riding comfort as well as the durability of the vehicle to deteriorate.
When the vehicle runs on a downwardly sloped path, it is not necessary to make a shift down operation even where the set switch is actuated for more speed. Moreover, when the vehicle runs on an upwardly sloped path, it is not necessary to make a shift down operation even where the coast switch is actuated for less speed.
Under such operating conditions, the shift down operation causes a hunting shift operation, based on repetitions of the shift down and up operations.